1. Field of the Invention
The present invention relates to an image heating apparatus for heating an image on a recording material, which is used for an image forming apparatus such as a copying machine, a printer, or a facsimile machine. Examples of the image heating apparatus include a fixing apparatus for fixing an unfixed image on a recording material and a glossiness increasing apparatus for increasing a glossiness of an image by heating the image fixed on a recording material.
2. Related Background Art
Conventionally, there has been a belt fixing apparatus in which a fixing roller and a belt are used, as described in Japanese Patent Application Laid-Open No. H11-045025.
In the belt fixing apparatus, it is possible to easily set a fixing nip width to be large along a direction of conveyance of the recording material by adjusting a contact width of the belt with respect to the fixing roller.
Therefore, the fixing nip width can be secured without depending on a diameter of the fixing roller, making it possible to reduce a diameter and a heat capacity of the fixing roller, to thereby shorten rise time of the belt fixing apparatus. That is, the belt fixing apparatus is capable of melting a large amount of toner. Therefore, the belt fixing apparatus is suitable for the image forming apparatus, which uses a large amount of toner such as a color image forming apparatus.
FIG. 9 is a schematic diagram of the belt fixing apparatus as described in Japanese Patent Application Laid-Open No. H11-045025 mentioned above. A fixing roller 51 is disposed such that the fixing roller 51 can be freely rotated. On a lower side of the fixing roller 51, a belt unit 53 is disposed.
Inside the fixing roller 51, a heater 58 such as a halogen lamp is disposed. In addition, a thermister (not shown) is disposed on the fixing roller 51. The thermister may be in contact with or out of contact with the fixing roller 51, and regulates a temperature of a surface of the fixing roller 51 by controlling a voltage supplied to the heater 58 via a temperature control circuit.
The belt unit 53 is provided with a fixing belt 52 which is a flexible endless belt; three rollers of a first roller 55, a second roller 56, and a third roller 57, which are plural winding and stretching members around which the fixing belt 52 is wound and stretched; a pressure pad 1000; and the like. The pressure pad 1000 is provided with a low friction sheet.
The separation roller 56 and the pressure pad 1000 are brought into pressure contact with the fixing roller 51 via the fixing belt 52, and thus a fixing nip portion N′ which is a contact portion of the fixing roller 51 and the fixing belt 52 are formed with a width large in the direction of conveyance of the recording material.
FIG. 10 shows an example of a pressure distribution of the pressure of the fixing nip portion along the direction of conveyance of the recording material, and a curve A shows a case of a conventional belt fixing apparatus. A curve B shows an example of the pressure distribution of the fixing nip portion of the fixing apparatus using a conventional pair of rollers. A curve C shows a case in which in the conventional roller fixing type fixing apparatus, a thickness of an elastic member layer of the fixing roller is made to be thick and an amount of deformation of the elastic member layer is increased by increasing the pressurizing force with a pressure roller in order to increase a nip width without increasing an outer diameter of the fixing roller as less as possible.
The belt fixing apparatus has a merit in making a nip width W1 much wider than a nip width W2 of the roller fixing type fixing apparatus. A nip width W3 of the roller fixing type fixing apparatus is somewhat wider than the nip width W2 as shown in the curve B. However, an effect obtained by enlarging the nip width is small, and in addition, the thick elastic member layer causes a heat capacity to be large, which is disadvantageous in view of energy conservation.
In the belt fixing apparatus, the separation roller 56 and the pressure pad 1000 are separated from each other as shown in FIG. 9. The pressure between the separation roller 56 and the pressure pad 1000, however, is not zero, as is understood from the pressure distribution of the fixing nip portion as shown in the curve A of FIG. 10, and the pressure of the separation roller 56 and the pressure of the pressure pad 1000 are continuous to each other through a tension of the fixing belt 52.
In the belt fixing apparatus, however, when a recording material, such as a coated paper sheet, having a low air permeability is used, there is another problem in that a portion in which the glossiness is low (haze: hereinafter referred to as uneven glossiness), as shown in FIG. 11, is formed in an icicle-like shape on a solid image.
In other words, in a case where the recording material having a low air permeability such as a coated paper sheet is used, an image disturbance occurs due to an expansion of air or generation of steam inside the fixing nip. The expansion of air or the generation of water vapor is caused by the following reasons: that is, air existing in an unfixed toner layer stagnates in a portion A′ in which the pressure in the nip is low, namely, a boundary region of the pressure pad and the separation roller; and the stagnated air partly forms an air gap on a boundary surface of the recording material and the fixing roller, and disturbs a toner image which is not completely fixed yet.
In a case where a plain paper sheet is used, the plain paper sheet has an air permeability higher than the air permeability of the coated paper sheet, so the air existing in the toner layer is retained within the plain paper sheet which is a porous member having a large capacity; or otherwise, the air escapes by passing the plain paper sheet. Therefore, it can be considered that the image disturbance does not often occur.
Further, in the case of a recording material having an especially low air permeability such as a thick coated paper sheet, a larger amount of air is likely to be stagnated in the portion A′ in which the pressure is decreased. Therefore, the problem of occurrence of the uneven glossiness described above becomes more serious.
Note that under an environment of high humidity in which the air contains high moisture content, the problem like this noticeably occurs.